How did Donald Trump go from being a fringe player in politics to the position he is in now? Why did Hillary Clinton’s attacks on Trump finally seem to click last month? Is there some level at which the presidential campaign makes sense?

Digging down into how people make decisions at a primitive level is the specialty of author Robert Cialdini, a guru to salesmen and marketers since the publication of his 1984 book “Influence.” In his new book “Pre-Suasion: A Revolutionary Way to Influence and Persuade,” he returns with more tips about how to slither your way into people’s minds and rearrange what you find there.

Donald Trump’s speaking bears the stamp of Cialdini’s techniques, which Barack Obama is rumored to have used on his path to the White House and which Hillary Clinton might also be studying. (Asked by the Financial Times whether he was advising Clinton, Cialdini took a “longish pause” and answered, “It’s my policy not to speak about any campaign that’s ongoing. The emotions are too deep.”)

Voters like to think they weigh all options and make informed choices, but savvy political figures such as Trump know how to instead play off their fears and desires.

Take the simplicity of Trump’s language: Pundits mock the elementary-school vocabulary, but it reaches people. “When people can process something with cognitive ease,” writes Cialdini, “they experience increased neuronal activity in the muscles of their face that produce a smile.” An analysis of the names of attorneys at top law firms found that the harder a lawyer’s name was to pronounce, the lower he tended to place in the firm’s hierarchy.

Are Trump’s speeches scattered, self-contradictory, vague? Maybe, but Cialdini is more interested in how words sound. “All mental activity,” he argues, “arises as patterns of association within a vast and intricate neural network. Influence attempts will be successful” if they conjure up associations that put people in a persuadable frame of mind. The associations that arise from Trumpian words like “winning,” “great,” “beautiful” and “terrific” persuade voters to feel good about him.

Scott Adams, the “Dilbert” cartoonist and trained hypnotist who has been perceptively analyzing Trump’s skills on his blog, calls Trump a “master persuader” who has effectively been using a hypnotic style on the public.

And Cialdini is the Dumbledore of this black magic. He bases his studies on real-world experience: He went out into the field with the top salesmen, charity fundraisers and publicists and studied how they closed their deals. He also looks at social-science research such as a Stanford study that found that changing a single word can reorient the way people think about crime.

Describe crime as a “beast” and it puts people in a conservative, punishment-oriented frame of mind. Call it a “virus,” though, and people turn to the softer, liberal approach of treating the underlying causes. Those who were primed with the “beast” concept were, by a 22-point margin, more likely to choose the punitive answer than those told to think of crime as a “virus.” That gap is larger than the measured gender gap (9 points) or even the gap between Democrats and Republicans (8 points).

Subtly exposing people to words like “win,” “attain,” “succeed” and “master” increases their test scores and also boosts the time they’re willing to spend on a given task, Cialdini notes.

‘Political writers cringe at Trump’s boasting about how successful and popular he is, but he is answering people’s craving for authority’

Political writers cringe at Trump’s boasting about how successful and popular he is, but he is answering people’s craving for authority: As a rich man, he seems to personify the answers to people’s economic unease. Cialdini notes that test subjects asked to make difficult economic decisions had much more difficulty coming to a conclusion on their own than they did than when told a given option was recommended by a prominent economist.

Moreover, Trump’s boasting hammers home the message that we should want to be part of his in-group. Telling people that all their neighbors are doing something, or that they risk missing out on a movement, is a potent tactic, which is why President Obama used “Yes We Can” effectively.

Clinton, for her part, is still trying to make “Stronger Together” stick. That phrase has unfortunate overtones of a mandate to shed individuality though, just as the dismal takeaway from her book, “It Takes a Village,” was that politicians should be in charge of everything, right down to your parenting choices.

What Clinton did best, especially during the Democratic convention, was to abandon policy points and go primitive, using Trump’s invocation of fear against him.

If Trump plays off fearing illegal immigrants and terrorists, she makes Trump himself the thing to fear: If he can’t control himself on Twitter, how can he be trusted with nukes? It’s political jiujitsu that uses Trump’s attention-capturing skill against him. His every off-beat comment is equated with a mushroom cloud over your house.

Neither Clinton nor Trump can dictate terms this fall, though. That’s where the media comes in. Cialdini points out that studies show the media aren’t so great at telling people what to think — but they’re excellent at getting people to think about whatever is on the news.

The candidate seen as having the best stance on whatever is driving the headlines gets a huge boost. We don’t know whether that will turn out to be hurricanes, terrorism or Colin Kaepernick.